In recent years, the research and development of various display units having a thin film transistor (TFT) device have been made. Since this TFT is space-saving, it is used as a transistor for driving the display unit of portable equipment such as a mobile telephone, notebook personal computer or PDS (personal digital assistant). This TFT is mostly made from a silicon-based semiconductor material typified by crystalline silicon and amorphous silicon. This is because it can be manufactured by using the conventional production process and technology of semiconductor devices.
However, since the processing temperature is 350° C. or higher when the semiconductor production process is used, a substrate on which a semiconductor can be formed is limited. Particularly, most glass and flexible substrates have a heat resistant temperature of 350° C. or lower, and it is difficult to manufacture TFT by using the conventional semiconductor production process.
Therefore, the research and development of a TFT device (oxide TFT) which is obtained from an oxide semiconductor material and can be manufactured at a low temperature are now under way. Since the oxide TFT can be formed at a low temperature, it can be formed on a glass substrate or a flexible substrate such as a plastic substrate. Therefore, a device which has not been existent can be manufactured at a low cost. Further, it can be used in RF tags and display units by making use of the transparency of the oxide material. The prior art is disclosed in Japanese Unexamined Patent Application Publication No. 2000-150900 and Japanese Unexamined Patent Application Publication No. 2005-268724.